Kinetic and Mechanistic Aspects of the Anthracene Photodimerization

We now address ourselves to the problem of accounting for the observations noted in Section 2.2a by building a mechanism for the dimerization of anthracene and related compounds. 

We saw in Section 2.1c that Huorescence spectra are routinely determined by monitoring the emission at a right angle to the exciting light. 

FIoure 2.24. (a) Right-angle method (6) front-surface method. 

This method is perfectly suitable for low concentrations of fluorescent materials. However, in order to study factors which affect the fluorescence quantum yield, such as molecular association or photochemical reactions, much higher concentrations than can be used in the right-angle fluorescence method are required. This follows from the fact that the 0 0 vibrational bands in the absorption and emission spectra often overlap. Therefore at relatively high concentrations light emitted at these overlapping wavelengths will be reabsorbed. 

Assuming for the moment that dimerization occurs from the excited singlet state, one can write the following simple mechanism -  

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